View Full Version : Lock-on Altitude?

Alex Whittingham
3rd Jul 2001, 22:01
Several JAA exam questions have referred to the lock-on altitude and the optimum altitude. Questions such as:

Both the lock-on altitude and the optimum altitude increase during the cruise. Which is the best level to fly at?

(a)The optimum altitude
(b)The lock-on altitude
(c)Between the lock-on altitude
(d)Just below the optimum altitude

Optimum altitude I thought I understood but what is the lock-on altitude?

4th Jul 2001, 06:35
I think that we all would love to know what a "lock on altitude" is ...... don't you just love the need for examiners to amuse themselves in the face of boredom ?

4th Jul 2001, 15:44
Considering the overall lack of question validation in the JAR mess, what's the bet that this 'lock on altitude' term is an english translation of a colloquialism used in the language of the question writer, to refer to 'optimum altitude'? http://www.pprune.org/ubb/NonCGI/redface.gif http://www.pprune.org/ubb/NonCGI/tongue.gif

4th Jul 2001, 18:50
The only thing I can possibly think they might be talking about is the MAX cruise altitude in the cruise page of most FMC's ???????? (I have never heard this term used) . Either way to quote Boeing 'Best fuel mileage for a given speed schedule is achieved at optimum altitude'.

Sorry I can't be of more help

Best rgds

4th Jul 2001, 20:39
Um, just guessing here Alex, but its not something to do with autopilot altitude hold mode is it?

As in the pressure datum which the autopilot will maintain or "lock-onto".

I know that on some ancient aircraft (eg BAe146) the altitude hold mode will allow the aircraft to climb 100-200 feet as aircraft speed increases due to idiosyncracies of the particular pitot/static system and the severe lack of a digital air data computer.

Just guessing though!

4th Jul 2001, 20:58
Another angle.

I am thinking about the atmosphere being "higher" or "thicker" at the equator than it is at the poles.

Consider a flight from the north pole to the equator. If the aircraft maintains a given flight level during the cruise its actual distance above mean sea level will increase as the aircraft progesses towards the equator. FL350 at the north pole is a lot closer to mean sea level than is FL350 at the euqator.

In this sense, although the aircraft autopilot may be "locked-on" to the pressure datum equivalent to FL350 its actual distance above mean sea level will increase.

Again, Alex, just guessing.

Capt Pit Bull
4th Jul 2001, 23:57
Hi Alex.

Perhaps its an Air Defence question? You know what these ex Navs are like.. ;-)


Nick Figaretto
5th Jul 2001, 00:44
I think parachuters use the term "lock on altitude" for the altitude wher they pull the string. Obviously it doesn't have anything to do with this...

Just a question, referring to alternative (c): How can you fly "between the lock-on altitude?"

I don't think I've never flown between an altitude before. :)


"I have found that alcohol taken in sufficient quantity produces all the effects of drunkenness."
~ Oscar Wilde

5th Jul 2001, 08:19

Re the autopilot's here and there wandering (and I have no knowledge of the 146) are you, perhaps, thinking of the altitude constrained phugoid which degenerates into a slowly oscillating airspeed indication ?

Alex Whittingham
5th Jul 2001, 10:25
Thanks guys, you confirmed what I thought, exam gobbledegook. Sorry, Nick, missed the last bit off answer (c). It was 'between the lock-on altitude and the optimum altitude'.

5th Jul 2001, 17:14
JT, no I did not mean to say that the autopilot altitude "wandered".

Rather that as aircraft indicated airspeed increases the altitude which the autopilot maintains whilst in altitude hold mode will increase by 100-200 feet, and then remain constant.

This is nothing to do with the pitch oscillation phugoids which are another feature of the 146, particularly in icing conditions.

As they say in Woodford, UK,

"Its not broken, its British!"